1. Field of the Invention
This invention relates to a process for the production of a network of drain pipes connected at the level of a panel for acoustic treatment.
2. Description of the Related Art
Techniques have been developed for reducing the noise emitted by an aircraft and in particular the noise emitted by a propulsion system, by using, at the walls of the pipes, panels (also called coatings or structures) whose purpose is to absorb a portion of the sound energy, in particular by using the principle of Helmholtz resonators. In a known manner, a panel for acoustic treatment comprises—from the outside to the inside—an acoustically resistive porous layer, at least one alveolar structure, and a reflective or impermeable layer.
Layer is defined as one or more layers that may or may not be of the same type.
The acoustically resistive porous layer is a porous structure that has a dissipative role, partially transforming into heat the acoustic energy of the sound wave passing through it. It comprises so-called open zones that are able to allow acoustic waves to pass and other so-called closed or filled zones that do not allow sound waves to pass but are designed to ensure the mechanical strength of said layer. This acoustically resistive layer is characterized in particular by an open surface area ratio that varies essentially based on the engine and components that constitute said layer.
A honeycomb can be used to form the alveolar structure. Different types of materials can be used to form the honeycomb.
A metal or composite panel can be used to form the reflective layer.
When the acoustic panel is installed at an air intake of a nacelle, this acoustic panel should also be compatible with a frost treatment. Frost treatment is defined as a process or a system that makes it possible to prevent the formation and/or the accumulation of ice and/or frost.
The documents EP-1,232,944 and EP-1,232,945 describe panels for acoustic treatment that is compatible with a frost treatment that uses hot air. In this case, the alveolar structure comes in the form of bands of cells that are spaced apart or a number of pipes spaced between them. This frost treatment is generally coupled to a drainage system that makes it possible to evacuate the water that may accumulate in the cells of the alveolar structure, in particular in the cells that are arranged in a zone that extends from 3 o'clock to 9 o'clock.
This drainage of water is necessary for limiting the risks of deterioration of the cells of the alveolar structure because of the gel of the accumulated water, for preventing corrosion problems, and for maintaining the acoustic performance levels.
According to one embodiment, the side walls of the cells comprise cut-outs in the top part or in the bottom part in such a way as to link cells together, the cells located at 6 o'clock or close to 6 o'clock, comprising openings at the reflective layer in such a way as to evacuate the water from the panel for acoustic treatment.
This embodiment is not completely satisfactory because the presence of a drainage system coupled to a system for frost treatment with hot air tends to reduce the capability of the acoustic treatment and to generate a stream perturbed at the stream of air entering the nacelle.
At the alveolar structure, the document FR-2,912,781 proposes producing a network of cells that are dedicated to a frost treatment communicating with one another and isolating one or more cells that are dedicated to acoustic treatment and isolated from the cells that are used to make the hot air circulate.
For draining the fluids, the reflective layer comprises openings, and pipes are provided at its free surface (opposite to the one against which the cells of the alveolar layer are attached).
To produce these pipes, a plate called a drainage plate below is applied against the free surface of the reflective layer with furrows that form pipes when said drainage plate is bonded against the free surface of the reflective layer.
So that the drainage is effective, it is necessary that the openings made in the reflective layer empty into the pipes.
However, these openings are generally made before deforming the panel for acoustic treatment. Consequently, after the shaping of the panel for adapting it to the radius of curvature of the nacelle, these openings generally are no longer aligned and their positions are relatively random.
If the drainage plate is attached after the shaping of the panel, it is difficult to determine the shape of the furrows so that they work with the openings.
If the drainage plate is attached before the shaping of the panel, with the deformations of the panel and the plate being random, it may occur that the openings that worked with the pipes before the shaping no longer cooperate afterwards.
According to another problem, the free surface of the reflective layer is not uniform and comprises hollow shapes of cells also called “telegraphing.” These surface defects reinforce the difficulty of obtaining airtight pipes for drainage.